Method of formulating and using a drilling mud with fragile gels

ABSTRACT

A method for drilling, running casing in, and/or cementing a borehole in a subterranean formation without significant loss of drilling fluid is disclosed, as well as compositions for use in such method. The method employs a drilling fluid comprising a fragile gel or having fragile gel behavior and providing superior oil mud rheology and overall performance. The fluid is especially advantageous for use in deep water wells because the fluid exhibits minimal difference between downhole equivalent circulating density and surface density notwithstanding differences in drilling or penetration rates. When an ester and isomerized olefin blend is used for the base of the fluid, the fluid makes an environmentally acceptable and regulatory compliant invert emulsion drilling fluid. The fluid preferably contains no organophilic clays.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/929,465, filed Aug. 14, 2001, pending, and acontinuation-in-part of International Patent Application Nos.PCT/US00/35609 and PCT/US00/35610, both filed Dec. 29, 2000, and pendingunder Chapter II of the Patent Cooperation Treaty.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to compositions and methods fordrilling, cementing and casing boreholes in subterranean formations,particularly hydrocarbon bearing formations. More particularly, thepresent invention relates to oil or synthetic fluid based drillingfluids and fluids comprising invert emulsions, such as, for example,fluids using esters, which combine high ecological compatibility withgood stability and performance properties.

[0004] 2. Description of Relevant Art

[0005] A drilling fluid or mud is a specially designed fluid that iscirculated through a wellbore as the wellbore is being drilled tofacilitate the drilling operation. The various functions of a drillingfluid include removing drill cuttings from the wellbore, cooling andlubricating the drill bit, aiding in support of the drill pipe and drillbit, and providing a hydrostatic head to maintain the integrity of thewellbore walls and prevent well blowouts. Specific drilling fluidsystems are selected to optimize a drilling operation in accordance withthe characteristics of a particular geological formation.

[0006] Oil or synthetic fluid-based muds are normally used to drillswelling or sloughing shales, salt, gypsum, anhydrite or other evaporiteformations, hydrogen sulfide-containing formations, and hot (greaterthan about 300 degrees Fahrenheit) holes, but may be used in other holespenetrating a subterranean formation as well. Unless indicatedotherwise, the terms “oil mud” or “oil-based mud or drilling fluid”shall be understood to include synthetic oils or other synthetic fluidsas well as natural or traditional oils, and such oils shall beunderstood to comprise invert emulsions.

[0007] Oil-based muds used in drilling typically comprise: a base oil(or synthetic fluid) comprising the external phase of an invertemulsion; a saline, aqueous solution (typically a solution comprisingabout 30% calcium chloride) comprising the internal phase of the invertemulsion; emulsifiers at the interface of the internal and externalphases; and other agents or additives for suspension, weight or density,oil-wetting, fluid loss or filtration control, and rheology control.Such additives commonly include organophilic clays and organophiliclignites. See H. C. H. Darley and George R. Gray, Composition andProperties of Drilling and Completion Fluids 66-67, 561-562 (5^(th) ed.1988). An oil-based or invert emulsion-based drilling fluid may commonlycomprise between about 50:50 to about 95:5 by volume oil phase to waterphase. An all oil mud simply comprises 100% oil by volume; that is,there is no aqueous internal phase.

[0008] Invert emulsion-based muds or drilling fluids comprise a keysegment of the drilling fluids industry. However, increasingly invertemulsion-based drilling fluids have been subjected to greaterenvironmental restrictions and performance and cost demands. There isconsequently an increasing need and industry-wide interest in newdrilling fluids that provide improved performance while still affordingenvironmental acceptance.

SUMMARY OF THE INVENTION

[0009] The present invention provides a fluid and a method for drillingboreholes or wellbores in subterranean formations with reduced loss ofdrilling fluids or muds into the formation. This advantage of theinvention is effected by formulating, providing or using a drillingfluid that forms a “fragile gel.” A “gel” may be defined a number ofways. One definition indicates that a “gel” is a generally colloidalsuspension or a mixture of microscopic water particles (and anyhydrophilic additives) approximately uniformly dispersed through the oil(and any hydrophobic additives), such that the fluid or gel has agenerally homogeneous gelatinous consistency. Another definition statesthat a “gel” is a colloid in a more solid form than a “sol” and definesa “sol” as a fluid colloidal system, especially one in which thecontinuous phase is a liquid. Still another definition provides that a“gel” is a colloid in which the disperse phase has combined with thecontinuous phase to produce a viscous jelly-like product. A gel has astructure that is continually building. If the yield stress of a fluidincreases over time, the fluid has gels. Yield stress is the stressrequired to be exerted to initiate deformation.

[0010] A “fragile gel” as used herein is a “gel” that is easilydisrupted or thinned, and that liquifies or becomes less gel-like andmore liquid-like under stress, such as caused by moving the fluid, butwhich quickly returns to a gel when the movement or other stress isalleviated or removed, such as when circulation of the fluid is stopped,as for example when drilling is stopped. The “fragileness” of the“fragile gels” of the present invention contributes to the unique andsurprising behavior and advantages of the present invention. The gelsare so “fragile” that it is believed that they may be disrupted by amere pressure wave or a compression wave during drilling. They seem tobreak instantaneously when disturbed, reversing from a gel back into aliquid form with minimum pressure, force and time and with lesspressure, force and time than known to be required to convert prior artfluids from a gel-like state into a flowable state.

[0011] When drilling is stopped while using a drilling fluid of thepresent invention, and consequently the stresses or forces associatedwith drilling are substantially reduced or removed, the drilling fluidforms a gel structure that allows it to suspend drill cuttings andweighting materials for delivery to the well surface. The drilling fluidof the invention suspends drill cuttings through its gel or gel-likecharacteristics, without need for organophilic clays to add viscosity tothe fluid. As a result, sag problems do not occur. Nevertheless, whendrilling is resumed, the fragile gel is so easily and instantlyconverted back into a liquid or flowable state that no initialappreciable or noticeable pressure spike is observed withpressure-while-drilling (PWD) equipment or instruments. In contrast,such pressure spikes are commonly or normally seen when using prior artfluids.

[0012] Further, the drilling fluid of the invention generally maintainsconsistently low values for the difference in its surface density andits equivalent density downhole during drilling operationsnotwithstanding variations in the rate of drilling or penetration intothe subterranean formation and notwithstanding other downhole stresseson the fluid. The fragile gels of the invention may be visco-elastic,contributing to their unique behavior and to the advantages of theinvention.

[0013] The drilling fluid of the invention responds quickly to theaddition of thinners, with thinning of the fluid occurring soon afterthe thinners are added, without need for multiple circulations of thefluid with the thinners additive or additives in the wellbore to showthe effect of the addition of the thinners. The drilling fluid of theinvention also yields flatter profiles between cold water and downholerheologies, making the fluid advantageous for use in deep water wells.That is, the fluid may be thinned at cold temperatures without causingthe fluid to be comparably thinned at higher temperatures. As usedherein, the terms “deep water” with respect to wells and “higher” and“lower” with respect to temperature are relative terms understood by oneskilled in the art of the oil and gas industry. However, generally, asused herein, “deep water wells” refers to any wells at water depthsgreater than about 1500 feet deep, “higher temperatures” meanstemperatures over about 120 degrees Fahrenheit and “lower temperatures”means temperatures at about 40 to about 60 degrees Fahrenheit. Rheologyof a drilling fluid is typically measured at about 120 or about 150degrees Fahrenheit.

[0014] A method for preparing and using a drilling fluid of theinvention is also provided by the invention. In the method, an invertemulsion drilling fluid is obtained or prepared that forms fragile gelsor that has fragile gel behavior, preferably without the addition oforganophilic clays or organophilic lignites, and that has as its base aninvert emulsion composition. An example of a suitable base is a blend ofesters with isomerized, or internal, olefins (“the ester blend”) asdescribed in U.S. patent application Ser. No. 09/929,465, of JeffKirsner (co-inventor of the present invention), Kenneth W. Pober andRobert W. Pike, filed Aug. 14, 2001, entitled “Blends of Esters withIsomerized Olefins and Other Hydrocarbons as Base Oils for InvertEmulsion Oil Muds, incorporated herein by reference.

[0015] Drilling fluids of the present invention prepared with such esterblends provide an invert emulsion drilling fluid having significantbenefits in terms of environmental acceptance or regulatory compliancewhile also improving oil mud rheology and overall oil mud performance.The esters in the blend may be any quantity, but preferably shouldcomprise at least about 10 weight percent to about 99 weight percent ofthe blend and the olefins should preferably comprise about 1 weightpercent to about 99 weight percent of the blend. The esters of the blendare preferably comprised of fatty acids and alcohols and most preferablyabout C₆ to about C₁₄ fatty acids and 2-ethyl hexanol. Esters made otherways than with fatty acids and alcohols, such as for example, estersmade from olefins combined with either fatty acids or alcohols, are alsobelieved effective.

[0016] Further, the invert emulsion drilling fluid has added to or mixedwith it other fluids or materials needed to comprise a complete drillingfluid. Such materials may include thinners or rheology control additivesfor example. However, preferably no organophilic clays are added to thedrilling fluid for use in the invention. Characterization of thedrilling fluid herein as “clayless” shall be understood to mean lackingorganophilic clays. Although omission of organophilic clays is a radicaldeparture from traditional teachings respecting preparation of drillingfluids, this omission of organophilic clays in the present inventionallows the drilling fluid to have greater tolerance to drill solids(i.e., the properties of the fluid are not readily altered by the drillsolids or cuttings) and is believed (without desiring to be limited bytheory) to contribute to the fluid's superior properties in use as adrilling fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIGS. 1(a), 1(b) and 1(c) provide three graphs showing field datacomparing mud losses incurred during drilling, running casing andcementing with a prior art isomerized olefin fluid and with a fluid ofthe present invention.

[0018]FIG. 1(a) shows the total downhole losses;

[0019]FIG. 1(b) shows the barrels lost per barrel of hole drilled; and

[0020]FIG. 1(c) shows the barrels lost per foot.

[0021]FIG. 2 is a graph comparing mud loss incurred running casing andcementing in seven boreholes at various depths, where the mud used inthe first three holes was a prior art isomerized olefin fluid and themud used in the last four holes was a fluid of the present invention.

[0022]FIG. 3 is a graph indicating gel formation in fluids of thepresent invention and their response when disrupted compared to someprior art isomerized olefin fluids.

[0023]FIG. 4 is a graph comparing the relaxation rates of various priorart drilling fluids and fluids of the present invention.

[0024]FIG. 5(a) is a graph comparing the differences in well surfacedensity and the equivalent circulating density for a prior artisomerized olefin fluid and for the fluid of the invention in twocomparable wells.

[0025]FIG. 5(b) shows the rate of penetration in the wells at the timethe density measurements for FIG. 5(a) were being taken.

[0026]FIG. 6 is a graph comparing the differences in well surfacedensity and the equivalent circulating density for a fluid of theinvention with a flowrate of 704 to 811 gallons per minute in a 12 ¼inch borehole drilled from 9,192 ft to 13,510 ft in deep water andincluding rate of penetration.

[0027]FIG. 7 is a graph comparing the differences in well surfacedensity and the equivalent circulating density for a fluid of theinvention with a flowrate of 158 to 174 gallons per minute in a 6 ½ inchborehole drilled from 12,306 ft to 13,992 ft and including rate ofpenetration.

[0028]FIG. 8 is a graph comparing the differences in well surfacedensity and the equivalent circulating density for a fluid of theinvention at varying drilling rates from 4,672 ft to 12,250 ft, and aflowrate of 522 to 586 gallons per minute in a 9 ⅞″ borehole.

[0029]FIG. 9(a) is a bar graph comparing the yield point of twodensities of a fluid of the invention at standard testing temperaturesof 40 and 120 degrees Fahrenheit.

[0030] FIGS. 9(b) and (c) are graphs of the Fann instrument dialreadings for these same two densities of a fluid of the invention over arange of shear rates at standard testing temperatures of 40 and 120degrees Fahrenheit.

[0031]FIG. 10 is a graph comparing the viscosity of various known invertemulsion bases for drilling fluids with the invert emulsion base for adrilling fluid of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0032] The present invention has been tested in the field and the fielddata demonstrates the advantageous performance of the fluid compositionof the invention and the method of using it. As illustrated in FIGS.1(a), (b), (c), and 2, the present invention provides an invert emulsiondrilling fluid that may be used in drilling boreholes or wellbores insubterranean formations, and in other drilling operations in suchformations (such as in casing and cementing wells), without significantloss of drilling fluid when compared to drilling operations with priorart fluids.

[0033] FIGS. 1(a), (b), and (c) show three graphs comparing the actualfluid loss experienced in drilling 10 wells in the same subterraneanformation. In nine of the wells, an isomerized olefin based fluid (inthis case, tradename PETROFREE® SF available from Halliburton EnergyServices, Inc. in Houston, Tex.), viewed as an industry “standard” forfull compliance with current environmental regulations, was used. In onewell, an ACCOLADE™ system, a fluid having the features orcharacteristics of the invention and commercially available fromHalliburton Energy Services, Inc. in Houston, Tex. (and also fullycomplying with current environmental regulations) was used. The holedrilled with an ACCOLADE™ system was 12.25 inches in diameter. The holesdrilled with the “standard” PETROFREE® SF fluid were about 12 inches indiameter with the exception of two sidetrack holes that were about 8.5inches in diameter. FIG. 1(a) shows the total number of barrels of fluidlost in drilling, running, casing and cementing the holes. FIG. 1(b)shows the total number of barrels of fluid lost per barrel of holedrilled. FIG. 1(c) shows the total number of barrels of fluid lost perfoot of well drilled, cased or cemented. For each of these wells graphedin these FIGS. 1(a), (b) and (c), the drilling fluid (or mud) lost whenusing a fluid of the invention was remarkably lower than when using theprior art fluid.

[0034]FIG. 2 compares the loss of mud with the two drilling fluids inrunning casing and cementing at different well depths in the samesubterranean formation. The prior art isomerized olefin based fluid wasused in the first three wells shown on the bar chart and a fluid of thepresent invention was used in the next four wells shown on the barchart. Again, the reduction in loss of fluid when using the fluid of thepresent invention was remarkable.

[0035] The significant reduction in mud loss seen with the presentinvention is believed to be due at least in substantial part to thefragile gel behavior of the fluid of the present invention and to thechemical structure of the fluid that contributes to, causes, or resultsin that fragile gel behavior. According to the present invention, fluidshaving fragile gels or fragile gel behavior provide significantreduction in mud loss during drilling (and casing and cementing)operations when compared to mud losses incurred with other drillingfluids that do not have fragile gel behavior. Without wishing to belimited by theory, it is believed, for example, that the structure ofthe drilling fluids of the invention, that is, the fragile gelstructure, contributing to the fragile gel behavior results in lowersurge and swab pressure while running casing which in turn results inlower mud loss during such casing operations. Thus, according to themethod of the invention, drilling fluid loss may be reduced by employinga drilling fluid in drilling operations that is formulated to comprisefragile gels or to exhibit fragile gel behavior. As used herein, theterm “drilling operations” shall mean drilling, running casing and/orcementing unless indicated otherwise.

[0036]FIG. 3 represents in graphical form data indicating gel formationin samples of two different weight (12.65 and 15.6 ppg) ACCOLADE® fluidsof the present invention and two comparably weighted (12.1 and 15.6 ppg)prior art invert emulsion fluids (tradename PETROFREE® SF) at 120degrees Fahrenheit. When the fluids are at rest or static (as whendrilling has stopped in the wellbore), the curves are flat or relativelyflat (see area at about 50-65 minutes elapsed time for example). Whenshear stress is resumed (as in drilling), the curves move up straightvertically or generally vertically (see area at about 68 to about 80elapsed minutes for example), with the height of the curve beingproportional to the amount of gel formed—the higher the curve the moregel built up. The curves then fall down and level out or begin to levelout, with the faster rate at which the horizontal line forms (and thecloser the horizontal line approximates true horizontal) indicating thelesser resistance of the fluid to the stress and the lower the pressurerequired to move the fluid.

[0037]FIG. 3 indicates superior response and performance by the drillingfluids of the present invention. Not only do the fluids of the presentinvention build up more gel when at rest, which enables the fluids ofthe invention to better maintain weight materials and drill cuttings insuspension when at rest—a time prior art fluids are more likely to havedifficulty suspending such solid materials—but the fluids of the presentinvention nevertheless surprisingly provide less resistance to thesheer, which will result in lower ECDs as will be discussed furtherbelow.

[0038]FIG. 4 provides data further showing the gel or gel-like behaviorof the fluids of the present invention. FIG. 4 is a graph of therelaxation rates of various drilling fluids, including fluids of thepresent invention and prior art isomerized olefin based fluids. In thetest, conducted at 120 degrees Fahrenheit, the fluids are exposed tostress and then the stress is removed. The time required for the fluidsto relax or to return to their pre-stressed state is recorded. Thecurves for the fluids of the invention seem to level out over timewhereas the prior art fluids continue to decline. The leveling out ofthe curves are believed to indicate that the fluids are returning to atrue gel or gel-like structure.

[0039] The significant reduction in mud loss seen with the presentinvention is also believed to be due in substantial part to thesuspected viscoelasticity of the fluid of the present invention. Suchviscoelasticity, along with the fragile gel behavior, is believed toenable the fluid of the invention to minimize the difference in itsdensity at the surface and its equivalent circulating density downhole.This difference in a drilling fluid's measured surface density at thewell head and the drilling fluid's equivalent circulating densitydownhole (as typically measured during drilling by downholepressure-while-drilling (PWD) equipment) is often called “ECD” in theindustry. Low “ECDs”, that is, a minimal difference in surface anddownhole equivalent circulating densities, is critical in drilling deepwater wells and other wells where the differences in subterraneanformation pore pressures and fracture gradients are small.

[0040] Table 1 below and FIG. 5(a) showing the Table 1 data in graphform illustrate the consistently stable and relatively minimaldifference in equivalent circulating density and actual mud weight orwell surface density for the fluids of the invention. This minimaldifference is further illustrated in FIG. 5(a) and in Table I by showingthe equivalent circulating density downhole for a commercially availableisomerized olefin drilling fluid in comparison to the drilling fluid ofthe present invention. Both fluids had the same well surface density.The difference in equivalent circulating density and well surfacedensity for the prior art fluid however was consistently greater thansuch difference for the fluid of the invention. FIG. 5(b) provides therates of penetration or drilling rates at the time the measurementsgraphed in FIG. 5(a) were made. FIG. 5(b) indicates that the fluid ofthe invention provided its superior performance—low ECDs—at surprisinglyfaster drilling rates, making its performance even more impressive, asfaster drilling rates tend to increase ECDs with prior art fluids. TABLE1 Comparison of Equivalent Circulating Densities PWD Data PWD DataACCOLADE ™ Isomerized Olefin System based fluid pump rate: 934 gpm MudWeight pump rate: 936 gpm DEPTH BIT: 12.25″ At well surface BIT: 12.25″(in feet) (ppg) (ppg) (ppg)  10600 12.29 12.0 12.51  10704 12.37 12.012.53  10798 12.52 12.0 12.72 10,899 12.50 12.2 12.70 11,001 12.50 12.212.64 11,105 12.52 12.2 12.70 11,200 12.50 12.2 12.69 11,301 12.55 12.212.70 11,400 12.55 12.2 12.71 11,500 12.59 12.2 12.77 11,604 12.59 12.212.79 11,700 12.57 12.2 12.79 11,802 12.60 12.2 12.79 11,902 12.62 12.212.81 12,000 12.64 12.2 12.83 12,101 12.77 12.2 12.99 12,200 12.77 12.312.99 12,301 12.76 12.3 13.01

[0041]FIG. 6 graphs the equivalent circulating density of an ACCOLADE™system, as measured downhole during drilling of a 12 ¼ inch boreholefrom 9,192 feet to 13,510 feet in deepwater (4,900 feet), pumping at 704to 811 gallons per minute, and compares it to the fluid's surfacedensity. Rate of penetration (“ROP”)(or drilling rate) is also shown.This data further shows the consistently low and stable ECDs for thefluid, notwithstanding differences in the drilling rate and consequentlythe differences in stresses on the fluid.

[0042]FIG. 7 similarly graphs the equivalent circulating density of anACCOLADE™ system, as measured downhole during drilling of a 6 ½ inchborehole from 12,306 feet to 13,992 feet, pumping at 158 to 174 gallonsper minute in deepwater, and compares it to the fluid's surface density.Rate of penetration (or drilling rate) is also shown. Despite therelatively erratic drilling rate for this well, the ECDs for thedrilling fluid were minimal, consistent, and stable. Comparing FIG. 7 toFIG. 6 shows that despite the narrower borehole in FIG. 6 (6 ½ inchescompared to the 12 ¼ inch borehole for which data is shown in FIG. 6),which would provide greater stress on the fluid, the fluid performanceis effectively the same.

[0043]FIG. 8 graphs the equivalent circulating density of an ACCOLADE™system, as measured downhole during drilling of a 9 ⅞ inch borehole from4,672 feet to 12,250 feet in deepwater, pumping at 522 to 585 gallonsper minute, and compares it to the surface density of the fluid and therate of penetration (“ROP”) (or drilling rate). The drilling fluidprovided low, consistent ECDs even at the higher drilling rates.

[0044] The present invention also provides a drilling fluid with arelatively flat rheological profile. Table 2 provides examplerheological data for a drilling fluid of the invention comprising 14.6pounds per gallon (“ppg”) of an ACCOLADE™ system. TABLE 2 ACCOLADE ™System Downhole Properties FANN 75 Rheology 14.6 lb/gal ACCOLADE ™System Temp. (° F.) 120 40 40 40 80 210 230 250 270 Pressure 0 0 34006400 8350 15467 16466 17541 18588 600 rpm 67 171 265 325 202 106 98 8982 300 rpm 39 90 148 185 114 63 58 52 48 200 rpm 30 64 107 133 80 49 4540 37 100 rpm 19 39 64 78 47 32 30 27 25 6 rpm 6 6 10 11 11 8 9 8 8 3rpm 5 6 10 11 11 8 9 8 8 Plastic 32 81 117 140 88 43 40 37 34 Viscosity(cP) Yield Point 7 9 31 45 26 20 18 15 14 (lb/100 ft²) N 0.837 0.9480.869 0.845 0.906 0.799 0.822 0.855 0.854 K 0.198 0.245 0.656 0.9450.383 0.407 0.317 0.226 0.21 Tau 0 (lb/100 ft² 4.68 6.07 8.29 8.12 9.687.45 8.21 8.29 7.75

[0045] FIGS. 9(b) and (c) compare the effect of temperature on pressuresobserved with two different fluid weights (12.1 and 12.4 ppg) whenapplying six different and increasing shear rates (2, 6, 100, 200, 300,and 600 rpm). Two common testing temperatures were used—40 and 120degrees Fahrenheit. The change in temperature and fluid weight resultedin minimal change in fluid behavior. FIG. 9(a) compares the yield pointof two different weight formulations (12.1 pounds per gallon and 12.4pounds per gallon) of a fluid of the present invention at two differenttemperatures (40 degrees Fahrenheit and 120 degrees Fahrenheit). Theyield point is unexpectedly lower at 40 degrees than at 120 degrees.Prior art oil-based fluids typically have lower yield points at highertemperatures, as traditional or prior art oils tend to thin or havereduced viscosity as temperatures increase. In contrast, the fluid ofthe invention can be thinned at lower temperatures without significantlyaffecting the viscosity of the fluid at higher temperatures. Thisfeature or characteristic of the invention is a further indicator thatthe invention will provide good performance as a drilling fluid and willprovide low ECDs. Moreover, this characteristic indicates the ability ofthe fluid to maintain viscosity at higher temperatures.

[0046]FIG. 10 compares the viscosity of a base fluid for comprising adrilling fluid of the present invention with known base fluids of someprior art invert emulsion drilling fluids. The base fluid for thedrilling fluid of the present invention is one of the thickest or mostviscous. Yet when comprising a drilling fluid of the invention, thedrilling fluid has low ECDs, provides good suspension of drill cuttings,satisfactory particle plugging and minimal fluid loss in use. Suchsurprising advantages of the drilling fluids of the invention arebelieved to be facilitated in part by a synergy or compatibility of thebase fluid with appropriate thinners for the fluid.

[0047] Thinners disclosed in International Patent Application Nos.PCT/US00/35609 and PCT/US00/35610 of Halliburton Energy Services, Inc.,Cognis Deutschland GmbH & Co KG., Heinz Muller, Jeff Kirsner(co-inventor of the present invention) and Kimberly Burrows (co-inventorof the present invention), both filed Dec. 29, 2000 and entitled“Thinners for Invert Emulsions,” and both incorporated herein byreference, are particularly useful in the present invention foreffecting such “selective thinning” of the fluid of the presentinvention; that is thinning at lower temperatures without rendering thefluid too thin at higher temperatures. Such thinners may have thefollowing general formula: R—(C₂H₄O)_(n)(C₃H₆O)_(m)(C₄H₈O)_(k)—H(“formula I”), where R is a saturated or unsaturated, linear or branchedalkyl radical having about 8 to about 24 carbon atoms, n is a numberranging from about 1 to about 10, m is a number ranging from about 0 toabout 10, and k is a number ranging from about 0 to about 10.Preferably, R has about 8 to about 18 carbon atoms; more preferably, Rhas about 12 to about 18 carbon atoms; and most preferably, R has about12 to about 14 carbon atoms. Also, most preferably, R is saturated andlinear.

[0048] The thinner may be added to the drilling fluid during initialpreparation of the fluid or later as the fluid is being used fordrilling or well service purposes in the formation. The quantity addedis an effective amount to maintain or effect the desired viscosity ofthe drilling fluid. For purposes of this invention, an “effectiveamount” of thinner of formula (I) is preferably from about 0.5 to about15 pounds per barrel of drilling fluid or mud. A more preferred amountof thinner ranges from about 1 to about 5 pounds per barrel of drillingfluid and a most preferred amount is about 1.5 to about 3 pounds thinnerper barrel of drilling fluid.

[0049] The compositions or compounds of formula (I) may be prepared bycustomary techniques of alkoxylation, such as alkoxylating thecorresponding fatty alcohols with ethylene oxide and/or propylene oxideor butylene oxide under pressure and in the presence of acidic oralkaline catalysts as is known in the art. Such alkoxylation may takeplace blockwise, i.e., the fatty alcohol may be reacted first withethylene oxide, propylene oxide or butylene oxide and subsequently, ifdesired, with one or more of the other alkylene oxides. Alternatively,such alkoxylation may be conducted randomly, in which any desiredmixture of ethylene oxide, propylene oxide and/or butylene oxide isreacted with the fatty alcohol.

[0050] In formula (I), the subscripts n and m respectively represent thenumber of ethylene oxide (EO) and propylene oxide (PO) molecules orgroups in one molecule of the alkoxylated fatty alcohol. The subscript kindicates the number of butylene oxide (BO) molecules or groups. Thesubscripts n, m, and k need not be integers, since they indicate in eachcase statistical averages of the alkoxylation. Included withoutlimitation are those compounds of the formula (I) whose ethoxy, propoxy,and/or butoxy group distribution is very narrow, such as for example,“narrow range ethoxylates” also called “NREs” by those skilled in theart.

[0051] To accomplish the purposes of this invention, the compound offormula (I) must contain at least one ethoxy group. Preferably, thecompound of formula I will also contain at least one propoxy group(C₃H₆O—) or butoxy group (C₄H₈O—). Mixed alkoxides containing all threealkoxide groups—ethylene oxide, propylene oxide, and butylene oxide—arepossible for the invention but are not preferred.

[0052] Preferably, for use according to this invention, the compound offormula (I) will have a value for m ranging from about 1 to about 10with k zero or a value for k ranging from about 1 to about 10 with mzero. Most preferably, m will be about 1 to about 10 and k will be zero.

[0053] Alternatively, such thinners may be a non-ionic surfactant whichis a reaction product of ethylene oxide, propylene oxide and/or butyleneoxide with C₁₀₋₂₂ carboxylic acids or C₁₀₋₂₂ carboxylic acid derivativescontaining at least one double bond in position 9/10 and/or 13/14 havingunits of the general formula:

[0054] (“formula II”) where R₁ is a hydrogen atom or an OH group or agroup OR₂, where R₂ is an alkyl group of about 1 to about 18 carbonatoms, or an alkenyl group of about 2 to about 18 carbon atoms or agroup of the formula:

[0055] where R₃ is a hydrogen atom, or an alkyl group of about 1 toabout 21 carbon atoms or an alkylene group of about 2 to about 21 carbonatoms. A formula (II) thinner may be used alone or may be used incombination with a formula (I) thinner or co-thinner.

[0056] Preferred commercially available thinners include, for example,products having the tradenames COLDTROL® (alcohol derivative), OMC2™(oligomeric fatty acid), ATC® (modified fatty acid ester), to be usedalone or in combination, and available from Halliburton Energy Services,Inc. in Houston, Tex.

[0057] The formulations of the fluids of the invention, and also theformulations of the prior art isomerized olefin based drilling fluids,used in drilling the boreholes cited in the data above, vary with theparticular requirements of the subterranean formation. Table 3 below,however, provides example formulations and properties for these twotypes of fluids discussed in the field data above. All trademarkedproducts in Table 3 are available from Halliburton Energy Services, Inc.in Houston, Tex., including: LE MUL™ emulsion stabilizer (a blend ofoxidized tall oil and polyaminated fatty acid); LE SUPERMUL™ emulsifier(polyaminated fatty acid); DURATONE® HT filtration control agent(organophilic leonardite); ADAPTA® HP filtration control agent(copolymer particularly suited for providing HPHT filtration control innon-aqueous fluid systems); RHEMOD L™ suspension agent/viscosifier(modified fatty acid); GELTONE® II viscosifier (organophilic clay);VIS-PLUS® suspension agent (carboxylic acid); BAROID® weighting agent(ground barium sulfate); and DEEP-TREAT® wetting agent/thinner(sulfonate sodium salt). In determining the properties in Table 3,samples of the fluids were sheared in a Silverson commercial blender at7,000 rpm for 10 minutes, rolled at 150 degrees Fahrenheit for 16 hours,and stirred for 10 minutes. Measurements were taken with the fluids at120 degrees Fahrenheit, except where indicated otherwise. TABLE 3Isomerized ACCOLADE ™ Olefin Based Invert Fluids and Compounds SystemEmulsion Drilling Fluid Example Formulations ACCOLADE ™ Base (bbl) 0.590— SF ™ Base (bbl) — 0.568 LE MUL ™¹ (lb) — 4 LE SUPERMUL ™² (lb) 10 6Lime (lb) 1 4 DURATONE ® HT³ (lb) — 4 Freshwater (bbl) 0.263 0.254ADAPTA ® HP⁴ (lb) 2 — RHEMOD L ™⁵ (lb) 1 — GELTONE ® II⁶ (lb) — 5VIS-PLUS ®⁷ (lb) — 1.5 BAROID ®⁸ (lb) 138 138 Calcium chloride (lb) 3231 DEEP-TREAT ®⁹ (lb) — 2 B. Properties Plastic Viscosity (cP) 19 19Yield Point (lb/100 ft²) 13 14 10 second gel (lb/100 ft²) 9 7 10 minutegel (lb/100 ft²) 12 9 HPHT Temperature (° F.) 225 200 HPHT @ 500 psid(mL) 0.8 1.2 Electrical stability (volts) 185 380 Fann ™ Dial Readings:600 rpm 51 52 300 rpm 32 33 200 rpm 25 26 100 rpm 18 18  6 rpm 7 7  3rpm 5 6

[0058] The present invention is directed to using invert emulsion baseddrilling fluids that contain fragile gels or exhibit fragile gelbehavior in drilling operations, such as drilling, running casing, andcementing. The present invention is also directed to reducing the lossof drilling fluids or drilling muds during such drilling operations byemploying invert emulsion based drilling fluids that contain fragilegels or exhibit fragile gel behavior, and that preferably provide lowECDs.

[0059] The invert emulsion drilling fluids of the present invention havean invert emulsion base. This base is not limited to a singleformulation. Test data discussed above is from example invert emulsiondrilling fluids of the invention comprising a blend of one or moreesters and one or more isomerized, or internal, olefins (“ester blend”)such as described in U.S. patent application Ser. No. 09/929,465, ofJeff Kirsner (co-inventor of the present invention), Kenneth W. Poberand Robert W. Pike, filed Aug. 14, 2001, entitled “Blends of Esters withIsomerized Olefins and Other Hydrocarbons as Base Oils for InvertEmulsion Oil Muds,” incorporated herein by reference. In such blend,preferably the esters will comprise at least about 10 weight percent ofthe blend and may comprise up to about 99 weight percent of the blend,although the esters may be used in any quantity. Preferred esters forblending are comprised of about C₆ to about C₁₄ fatty acids andalcohols, and are particularly or more preferably disclosed in U.S. Pat.Re. No. 36,066, reissued Jan. 25, 1999 as a reissue of U.S. Pat. No.5,232,910, assigned to Henkel KgaA of Dusseldorf, Germany, and BaroidLimited of London, England, and in U.S. Pat. No. 5,252,554, issued Oct.12, 1993, and assigned to Henkel Kommanditgesellschaft auf Aktien ofDusseldorf, Germany and Baroid Limited of Aberdeen, Scotland. Estersdisclosed in U.S. Pat. No. 5,106,516, issued Apr. 21, 1992, and U.S.Pat. No. 5,318,954, issued Jun. 7, 1984, both assigned to HenkelKommanditgesellschaft auf Aktien, of Dusseldorf, Germany, may also beused. The most preferred esters for use in the invention are comprisedof about C₁₂ to about C₁₄ fatty acids and 2-ethyl hexanol or about C₈fatty acids and 2-ethyl hexanol. These most preferred esters areavailable commercially under tradenames PETROFREE® and PETROFREE LV™,respectively, from Halliburton Energy Services, Inc. in Houston, Tex.Although esters made with fatty acids and alcohols are preferred, estersmade other ways, such as from combining olefins with either fatty acidsor alcohols, may also be effective.

[0060] Isomerized, or internal, olefins for blending with the esters foran ester blend may be any such olefins, straight chain, branched, orcyclic, preferably having about 10 to about 30 carbon atoms. Isomerized,or internal, olefins having about 40 to about 70 weight percent C₁₆ andabout 20 to about 50 weight percent C₁₈ are especially preferred. Anexample of an isomerized olefin for use in an ester blend in theinvention that is commercially available is SF™ Base fluid, availablefrom Halliburton Energy Services, Inc. in Houston, Tex. Alternatively,other hydrocarbons such as paraffins, mineral oils, glyceride triesters,or combinations thereof may be substituted for or added to the olefinsin the ester blend. Such other hydrocarbons may comprise from about 1weight percent to about 99 weight percent of such blend.

[0061] Invert emulsion drilling fluids may be prepared comprising SF™Base without the ester, however, such fluids are not believed to providethe superior properties of fluids of the invention with the ester. Fielddata discussed above has demonstrated that the fluids of the inventionare superior to prior art isomerized olefin based drilling fluids, andthe fluids of the invention have properties especially advantageous insubterranean wells drilled in deep water. Moreover, it is believed thatthe principles of the method of the invention may be used with invertemulsion drilling fluids that form fragile gels or yield fragile gelbehavior, provide low ECDs, and have (or seem to have) viscoelasticitythat may not be comprised of an ester blend. One example of such a fluidmay comprise a polar solvent instead of an ester blend.

[0062] Other examples of possible suitable invert emulsion bases for thedrilling fluids of the present invention include isomerized olefinsblended with other hydrocarbons such as linear alpha olefins, paraffins,or naphthenes, or combinations thereof (“hydrocarbon blends”).

[0063] Paraffins for use in blends comprising invert emulsions fordrilling fluids for the present invention may be linear, branched,poly-branched, cyclic, or isoparaffins, preferably having about 10 toabout 30 carbon atoms. When blended with esters or other hydrocarbonssuch as isomerized olefins, linear alpha olefins, or naphthenes in theinvention, the paraffins should comprise at least about 1 weight percentto about 99 weight percent of the blend, but preferably less than about50 weight percent. An example of a commercially available paraffinsuited for blends useful in the invention is called tradename XP-07™,available from Halliburton Energy Services, Inc. in Houston, Tex. XP-07™is primarily a C₁₂₋₁₆ linear paraffin.

[0064] Examples of glyceride triesters for ester/hydrocarbon blendsuseful in blends comprising invert emulsions for drilling fluids for thepresent invention may include without limitation materials such asrapeseed oil, olive oil, canola oil, castor oil, coconut oil, corn oil,cottonseed oil, lard oil, linseed oil, neatsfoot oil, palm oil, peanutoil, perilla oil, rice bran oil, safflower oil, sardine oil, sesame oil,soybean oil, and sunflower oil.

[0065] Naphthenes or napthenic hydrocarbons for use in blends comprisinginvert emulsions for drilling fluids for the present invention may beany saturated, cycloparaffinic compound, composition or material with achemical formula of C_(n)H_(2n) where n is a number about 5 to about 30.

[0066] In still another embodiment, a hydrocarbon blend might be blendedwith an ester blend to comprise an invert emulsion base for a drillingfluid of the present invention.

[0067] The exact proportions of the components comprising an ester blend(or other blend or base for an invert emulsion) for use in the presentinvention will vary depending on drilling requirements (andcharacteristics needed for the blend or base to meet thoserequirements), supply and availability of the components, cost of thecomponents, and characteristics of the blend or base necessary to meetenvironmental regulations or environmental acceptance. The manufactureof the various components of the ester blend (or other invert emulsionbase) will be understood by one skilled in the art.

[0068] Further, the invert emulsion drilling fluid of the invention orfor use in the present invention has added to it or mixed with theinvert emulsion base, other fluids or materials needed to comprise acomplete drilling fluid. Such materials may include for exampleadditives to reduce or control temperature rheology or to providethinning, such as, for example, additives having the tradenamesCOLDTROL®, RHEMOD L™, ATC®, and OMC2™; additives for providing temporaryincreased viscosity for shipping (transport to the well site) and foruse in sweeps, such as, for example an additive having the tradenameTEMPERUS™ (modified fatty acid); additives for filtration control, suchas, for example additives having the tradename ADAPTA HP®; additives forhigh temperature high pressure control (HTHP) and emulsion stability,such as, for example, additives having the tradename FACTANT™ (highlyconcentrated tall oil derivative); and additives for emulsification,such as, for example additives having the tradename LE SUPERMUL™(polyaminated fatty acid). Blends of thinners such as the OMC2™,COLDTROL®, and ATC® thinners can be more effective in fluids of theinvention than a single one of these thinners. All of the aforementionedtrademarked products are available from Halliburton Energy Services,Inc. in Houston, Tex., U.S.A.

[0069] The invert emulsion drilling fluid of the present inventionpreferably does not have added to it any organophilic clays. The fluidof the invention does not need organophilic clay or organophiliclignites to provide it needed viscosity, suspension characteristics, orfiltration control to carry drill cuttings to the well surface.Moreover, the lack of organophilic clays and organophilic lignites inthe fluid is believed to enhance the tolerance of the fluid to the drillcuttings. That is, the lack of organophilic clays and organophiliclignites in the fluid of the invention is believed to enable the fluidto suspend and carry drill cuttings without significant change in thefluid's rheological properties.

[0070] The foregoing description of the invention is intended to be adescription of preferred embodiments. Various changes in the details ofthe described fluids and method of use can be made without departingfrom the intended scope of this invention as defined by the appendedclaims.

We claim:
 1. A method for conducting a drilling operation in asubterranean formation using a fragile gel drilling fluid comprising:(a) an invert emulsion base; (b) one or more thinners; (c) one or moreemulsifiers; and (d) one or more weighting agents.
 2. The method ofclaim 1 wherein said invert emulsion has a continuous base comprised ofat least one component selected from at least one of the followinggroups: (i) esters prepared from fatty acids and alcohols, estersprepared from olefins and fatty acids or alcohols; (ii) olefinscomprising linear alpha olefins, isomerized olefins having a straightchain olefins having a branched structure, isomerized olefins having acyclic structure; olefin hydrocarbons; (iii) paraffin hydrocarbonscomprising linear paraffins, branched paraffins, poly-branchedparaffins, cyclic paraffins, isoparaffins; (iv) mineral oilhydrocarbons; (v) glyceride triesters comprising rapeseed oil, oliveoil, canola oil, castor oil, coconut oil, corn oil, cottonseed oil, lardoil, linseed oil, neatsfoot oil, palm oil, peanut oil, perilla oil, ricebran oil, safflower oil, sardine oil, sesame oil, soybean oil, sunfloweroil; and (vi) naphthenic hydrocarbons.
 3. The method of claim 2 whereinsaid thinner is selected from at least one of the following groups: (i)a compound having the formula: R—(C₂H₄O)_(n)(C₃H₆O)_(m)(C₄H₈O)_(k)—H where R is a saturated or unsaturated, linear or branched alkyl radicalhaving about 8 to about 24 carbon atoms, n is a number ranging fromabout 1 to about 10, m is a number ranging from about 0 to about 10, andk is a number ranging from about 0 to about 10; and (ii) a non-ionicsurfactant, said surfactant being the reaction product of at least oneoxide, selected from the group comprising ethylene oxide, propyleneoxide and butylene oxide, with C₁₀₋₂₂ carboxylic acids or C₁₀₋₂₂carboxylic acid derivatives containing at least one double bond inposition 9, 10, 13, or 14 having structural units of the generalformula:

 where R₁ is a hydrogen atom, or an OH group, or a group OR_(2,) andwhere R₂ is an alkyl group of about 1 to about 18 carbon atoms, or analkenyl group of about 2 to about 18 carbon atoms, or a group of theformula:

 and where R₃ is a hydrogen atom, or an alkyl group of about 1 to about21 carbon atoms, or an alkylene group of about 2 to about 21 carbonatoms.
 4. The method of claim 1 wherein said operation is drilling aborehole.
 5. The method of claim 1 wherein said operation is runningcasing in a borehole.
 6. The method of claim 1 wherein said operation iscementing a borehole.
 7. The method of claim 1 wherein said fragile gelis a structure capable of suspending drill cuttings that may beimmediately disrupted by movement of said fluid.
 8. The method of claim4 wherein said fragile gel reverts to a flowable or liquid stateimmediately upon resumption of drilling after a period of rest.
 9. Themethod of claim 4 wherein no appreciable pressure spike is observed bypressure-while-drilling equipment when said drilling is resumed.
 10. Themethod of claim 1 wherein said thinner reduces the viscosity of saiddrilling fluid at lower temperatures to a greater extent than it reducesthe viscosity of said drilling fluid at higher temperatures.
 11. Themethod of claim 1 wherein said drilling fluid has a lower yield point atlower temperatures than at higher temperatures.
 12. The method of claim1 wherein said drilling fluid is visco-elastic.
 13. The method of claim1 wherein the equivalent circulating density of said drilling fluidapproximates the surface density of said drilling fluid.
 14. The methodof claim 4 wherein the ECDs are less than about 0.5.
 15. The method ofclaim 14 wherein said drilling is in deep water
 16. The method of claim14 wherein said fluid is tolerant to contaminants, wherein saidcontaminants comprise drill cuttings from said drilling.
 17. The methodof claim 1 wherein said fluid does not exhibit sag when at rest.
 18. Afragile gel drilling fluid comprising: (a) an invert emulsion base; (b)one or more thinners; (c) one or more emulsifiers; and (d) one or moreweighting agents.
 19. The drilling fluid of claim 18 wherein said invertemulsion has a continuous base comprised of at least one componentselected from at least one of the following groups: (i) esters preparedfrom fatty acids and alcohols, esters prepared from olefins and fattyacids or alcohols; (ii) olefins comprising linear alpha olefins,isomerized olefins having a straight chain olefins having a branchedstructure, isomerized olefins having a cyclic structure; olefinhydrocarbons; (iii) paraffin hydrocarbons comprising linear paraffins,branched paraffins, poly-branched paraffins, cyclic paraffins,isoparaffins; (iv) mineral oil hydrocarbons; (v) glyceride triesterscomprising rapeseed oil, olive oil, canola oil, castor oil, coconut oil,corn oil, cottonseed oil, lard oil, linseed oil, neatsfoot oil, palmoil, peanut oil, perilla oil, rice bran oil, safflower oil, sardine oil,sesame oil, soybean oil, sunflower oil; and (vi) naphthenichydrocarbons.
 20. The drilling fluid of claim 19 wherein said thinner isselected from at least one of the following groups: (i) a compoundhaving the formula: R—(C₂H₄O)_(n)(C₃H₆O)_(m)(C₄H₈O)_(k)—H  where R is asaturated or unsaturated, linear or branched alkyl radical having about8 to about 24 carbon atoms, n is a number ranging from about 1 to about10, m is a number ranging from about 0 to about 10, and k is a numberranging from about 0 to about 10; and (ii) a non-ionic surfactant, saidsurfactant being the reaction product of at least one oxide, selectedfrom the group comprising ethylene oxide, propylene oxide and butyleneoxide, with C₁₀₋₂₂ carboxylic acids or C₁₀₋₂₂ carboxylic acidderivatives containing at least one double bond in position 9, 10, 13,or 14 having structural units of the general formula:

 where R₁ is a hydrogen atom, or an OH group, or a group OR_(2,) andwhere R₂ is an alkyl group of about 1 to about 18 carbon atoms, or analkenyl group of about 2 to about 18 carbon atoms, or a group of theformula:

 and where R₃ is a hydrogen atom, or an alkyl group of about 1 to about21 carbon atoms, or an alkylene group of about 2 to about 21 carbonatoms.
 21. The drilling fluid of claim 19 wherein said drilling fluidcomprises a blend of said esters and said olefins, and said thinners andemulsifiers are derived from fatty acids, wherein said esters areselected from the group comprising about C₁₂ to about C₁₄ fatty acidsand 2-ethyl hexanol or about C₈ fatty acids and 2-ethyl hexanol and saidolefins are selected from the group comprising isomerized, or internal,olefins having about 40 to about 70 weight percent C₁₆ and about 20 toabout 50 weight percent C₁₈ and wherein said thinners are selected fromthe group comprising: thinners having the following general formula:R—(C₂H₄O)_(n)(C₃H₆O)_(m)(C₄H₈O)_(k)—H, where R is a saturated, linearalkyl radical having about 12 to about 14 carbon atoms, n is a numberranging from about 1 to about 10, m is a number ranging from about 0 toabout 10, and k is a number ranging from about 0 to about 10; thinnerscomprising a non-ionic surfactant which is a reaction product ofethylene oxide, propylene oxide and/or butylene oxide with C₁₀₋₂₂carboxylic acids or C₁₀₋₂₂ carboxylic acid derivatives containing atleast one double bond in position 9/10 and/or 13/14 having units of thegeneral formula:

 where R₁ is a hydrogen atom or an OH group or a group OR₂, where R₂ isan alkyl group of about 1 to about 18 carbon atoms, or an alkenyl groupof about 2 to about 18 carbon atoms or a group of the formula:

 where R₃ is a hydrogen atom, or an alkyl group of about 1 to about 21carbon atoms or an alkylene group of about 2 to about 21 carbon atoms;and combinations thereof; and wherein said drilling fluid furthercomprises at least one additional additive compatible with said esterblend wherein said additive is selected from the group comprisingadditives for filtration control, additives for high temperature highpressure control, and additives to reduce or control temperaturerheology.
 22. The drilling fluid of claim 18 wherein said drilling fluidforms a structure that is capable of suspending drill cuttings at restand that is instantaneously disruptible by movement.
 23. The drillingfluid of claim 18 wherein said fluid forms a fragile gel at rest. 24.The drilling fluid of claim 18 wherein the ECDs of said fluid when usedin drilling boreholes are less than about 0.5.
 25. The drilling fluid ofclaim 18 wherein said fluid is visco-elastic.
 26. A method for reducingthe loss of drilling fluid into a subterranean formation while drillinga borehole in said formation, said method comprising using the drillingfluid of claim
 18. 27. A method for eliminating or avoiding sag duringdrilling operations, said method comprising employing the drilling fluidof claim 18 in said drilling operations.
 28. A method for reducing theloss of drilling fluid into a subterranean formation while drilling aborehole in said formation, said method comprising using the drillingfluid of claim
 18. 29. A method for preparing a drilling fluidcomprising formulating a drilling fluid comprising an invert emulsionbase comprising an ester blend and further comprising a thinnercompatible with said ester blend such that said fluid builds a structureat rest capable of suspending drill cuttings without appreciable sag andwherein said structure may be immediately disrupted by movement of saidfluid without registering an appreciable pressure spike withpressure-while-drilling equipment.